One type of transversely excited atmospheric laser is described in "IMPROVED EXCITATION TECHNIQUES FOR ATMOSPHERIC PRESSURE CO.sub.2 LASER" in "ELECTRONICS LETTERS", Mar. 25, 1971, Vol. 7 Nos. 516. The transversely excited atmospheric laser comprises main electrodes including an anode and cathode having a predetermined length in a longitudinal direction respectively and facing each other with a predetermined gap in parallel, and preliminary discharge electrodes provided in the vicinity of the main electrodes. In the transversely excited atmospheric laser, a high voltage is applied across the preliminary electrodes to produce arc discharge so that ultraviolet light induced by the arc discharge is radiated to the discharge space between the anode and cathode electrodes of the main electrodes, thereby resulting in the ionization of laser gas. As a result, a uniform glow discharge is developed when a high voltage is applied across the main electrodes.
In the transversely excited atmospheric laser, however, the main discharge space between the anode and cathode electrodes and preliminary discharge space between the preliminary discharge electrodes are provided in a single chamber so that laser gas therein is dissociated due to the intensive arc discharges of the preliminary electrodes, thereby shortening the life term of laser gas extremely. Further, it is impossible to control the wavelength of preliminary discharge light, rise time etc. independently because the discharge for preliminary ionization is performed in the same conditions as those of laser gas in the main discharge.
In "Investigations of glow discharge formation with volume preionization" on pages 1567 to 1574 of "Journal of Applied Physics", Vol. 46, No. 4, April 1975, there is described that electron density N.sub.e (r) resulting from preliminary ionization at a point from which an arc discharge source is located with a distance r is expressed in the equation (1). EQU N.sub.e (r)=S.sub.e (N.sub.r)N/r.sup.2 ( 1)
where S.sub.e (N.sub.r) is the number of photoelectrons per sr Torr cm as a function of the spark parameters, and is determined by geometrical positions of experiment instruments or tools, and N is a molecular number.
Electron density varies as the inverse square of the distance r from a preliminary discharge source. For this reason, it is preferable to adopt a construction in which the preliminary discharge electrodes are as near the main discharge electrodes as possible to promote uniform and intensive preliminary ionization in the main discharge space.
If the preliminary discharge electrodes are made nearer the main discharge electrodes, however, discharge energy which is necessary for the ionization of laser gas is not sufficiently supplied to the main discharge space because unexpected discharge tends to occur at unexpected portions between the preliminary discharge electrodes and the main discharge electrodes.